The present invention relates to telephone circuit for generating signals designed to inform the subscriber of the relative charge of a telephone call in progress, and in particular to telephone circuits, which may be monolithically integrated, and which generate control signals for the display of the charge to the subscriber, and which are designed to form, together with the speech circuit of the subscriber's line, an interface between the subscriber's telephone line and exchange control components at the telephone exchange.
A subscriber's telephone line is supplied by a direct voltage generator, connected in series with other voltage signal generators, for example, speech band voice signals, ringing signals and signals for the subscriber's telephone charge display.
Both the speech circuit and the ringing circuit of the subscriber's telephone set are connected to terminals of the line in parallel with one another, as well as to other devices, both internal to and external to the telephone set, for example, a display for the progressive metering of the charge of the telephone call in question which is calculated by appropriate exchange components.
The subscriber's telephone charge displays are, as mentioned above, driven by the telephone exchange, via the same telephone line using suitable control signals, normally called "charge signals".
These control signals must have a high amplitude with respect to the maximum amplitude of the speech signals, but cannot take the form of signals having a pulse waveshape as in this case the leading and trailing edges of the pulses would cause harmonics likely not only to disturb the line subscriber but also the subscribers of adjacent lines. In practice, use is made of alternating voltage signals, having a duration which is limited over time (approximately 100 msec) and a frequency (12 or 16 KHz) which is considerably greater than the frequency of the telephone speech signals (0.3 to 3.4 KHz), with relatively long signal level attack and decay times.
At present, the "charge signals" are generally formed and supplied directly to the line by exchange components of the electromechanical type comprising a resonant circuit coupled inductively, via a transformer, to the subscriber's telephone line and activated for predetermined time intervals.
The curve of the increase and decrease in the signal level is of the exponential type.
The above solution is therefore comparatively costly and makes it necessary to tune the resonant circuit accurately to the required frequency.
Copending U.S. patent application Ser. No. 731,167, filed May 6, 1985, now U.S. Pat. No. 4,638,122 sets out a solution which is more suited to the requirements of modern telephone systems of electronic type, and discloses a telephone circuit which generates control signals for the subscriber's telephone charge display and is designed to be monolithically integrated and to form, together with the speech circuit of the subscriber's line, an interface between the subscriber's telephone line and control components at the exchange. This telephone circuit is coupled to a generator of AC voltage signals having a predetermined amplitude and frequency which are constant over time, and comprises a generator of voltage signals which are spaced over time and have a trapezoidal pulse waveshape.
A multiplier circuit calculates the product of the signals supplied by the two generators and supplies a signal which is sent to the speech circuit of the subscriber's line and is added to the speech signals.
It also comprises: a circuit means designed to take an image signal of the aggregate signal already adapted to the line; a high-pass filter designed to eliminate the speech signal components of the image signal; a rectifier circuit designed to rectify the filtered signal from the filter and a comparator designed to compare the rectified signal with a reference signal.
If the amplitude of the rectified signal is greater than the amplitude of the reference signal, the comparator generates a signal designed to stop the increase in the level of the trapezoidal pulse signals, thereby regulating their amplitude.
The generator of the trapezoidal pulse signals is constructed using a circuit comprising a capacitor having a first terminal which is connected to ground, or to an equivalent reference potential, and having a second terminal which is coupled to a supply source, having a positive potential with respect to the ground potential, via a first controlled switch and a first constant current generator connected in series.
The second terminal of the capacitor is then coupled to ground or to a negative potential reference with respect to the ground potential via a second switch and a second constant current generator connected in series.
The first and second switches are caused to close by the exchange control components, whilst the first switch is caused to open by the signal generated by the comparator.
The discharge of the capacitor is indicated by suitable circuit means which cause the second switch to open when the voltage across the terminal of the capacitor is zero.
The voltage across the terminals of the capacitor is designed to constitute the signal supplied by the pulse signal generator.
The capacitor also acts as a memory element for keeping the signal level constant when both switches are open.